Various engines whose intakes are provided with closing flaps have been known. For example, the Applicant's of DE-PS 42 35 631 describe a cross section-controlled rectangular air intake, in which a pivotable adjusting ramp, which makes it possible to greatly vary the inlet cross section and consequently the air throughput, is located in the lower part of the intake between plane-parallel side walls. A diffuser part located between the adjusting ramp and an upper, fixed shock wave ramp makes it possible to absorb the inlet head wave in the case of a small internal cross section contraction and consequently to build up an internal supersonic compression. The lower closure of the flow is guaranteed by an auxiliary flap, which extends under the adjusting ramp to a connection web arranged between the side walls. This air intake makes it possible to achieve a low-resistance throughput control at the lowest possible external resistance for the entire range of Mach numbers.
The external and internal geometry of the air intake not only for engines of airplanes but also for engines of missiles is set by the operating conditions. It is, in general, absolutely necessary, especially in the case of missiles started from the air, to close the air inlet before the start of the engine by displaceable parts of the air intake or by closing flaps.